It is well known that the hearth of an iron blast furnace is provided with a tap hole, commonly referred to as an "iron notch" through which molten iron, usually referred to as "hot metal", is drawn off at periodic intervals during a blast furnace campaign. During a normal campaign, such tapping must be done an average of five to twelve times daily, as the blast furnace hearth becomes filled with molten iron (i.e., hot metal) and molten slag. After the blast furnace has been tapped, i.e. the molten hot metal and slag drained therefrom, the tap hole, or iron notch, is plugged with clay or "mud" which will harden and seal to tap hole until the next time the blast furnace is tapped.
In accordance with conventional practices, a special drill is often utilized to open the tap hole, i.e., drill a passageway through the hardened clay plugging the iron notch, for purposes of tapping the blast furnace. Such blast furnace tap hole drills are normally pneumatically or hydraulically operated rotary-percussion drills comparable to rock drills utilized in the mining industry. Such drills impart both a rotary and an impact force on an elongated drill rod having a rock drill bit at the end thereof, which is disposed against the iron notch, and normally includes a compressed air system for blowing compressed air through the center of the drill bit and drill rod assembly to purge drilling debris from the drilled hole. Typical operations usually utilize a four-wing drill bit having an axial air passage through which compressed air is blown, with the drill bit attached to an elongated drill rod (typically at least about 10 feet in length) having an air passageway drilled throughout the full axial length thereof.
The base support for the blast furnace tap hole drill is normally secured to the floor, a structural column or some solid base structure in the vicinity of the blast furnace iron notch, and is provided with suitable linkage members and remote controls so that the blast furnace tap hole drill can be remotely operated to move the drill into proper position for drilling the tap hole, then operated to drill the tap hole, and thereafter moved back away from the tap hole and the heat of the emerging hot metal, where the drill can be serviced and prepared for the next tap.
To prepare the blast furnace tap hole drill for each succeeding tap, it is always necessary to replace the drill bit, if not the entire drill bit and drill rod assembly to which the drill bit is secured. This is because the temperature of the blast furnace hot metal, being about 2700-2800.degree. F., severely erodes the drill bit after it drills through the clay plug and enters the bath of molten hot metal. In addition, once the tap hole is drilled, the ferrostatic head of hot metal, within the blast furnace, will cause hot metal to emerge through the tap hole around the drill bit and drill rod before the drill rod and bit can be safely withdrawn from the tap hole. Oftentimes, the drill bit and drill rod will not only be severely eroded, but portions of the drill bit remaining may virtually be "welded" to the end of the drill rod to which it had previously been removably attached. In such event, it may be impossible to remove the drill bit from the drill rod to replace a new drill bit, and accordingly, it usually becomes necessary to replace the entire assembly, i.e., drill bit and adjoining drill rod or drill rod component to which the drill bit becomes welded. Accordingly, the expense associated with such a result is normally rather excessive when considering not only the cost of the replacement drill bit, but also the cost of the replacement drill rod, both of which are normally beyond salvage and must be discarded. Indeed, such drill rods, as necessary for tapping shaft furnaces, such as a blast furnace, are rather costly to produce in that they must be good quality steel of exceptional length, and must have an air hole drilled through the full length. Accordingly, the art is not only in need of a lower cost drill bit, but also in need of a lower cost drill rod, one in which the costly need to drill an elongated, axial air passageway can be eliminated or reduced.